The invention is in the field of polymers containing vinyl fluoride units, their preparation and use in making protective coatings and films.
Polyvinyl fluoride (PVF) has been manufactured for many years and has found many uses as a protective film or coating over a variety of substrates because of its excellent weatherability, chemical resistance and mechanical properties. The fluorine atoms in PVF are largely responsible for these properties and, if polymers of higher fluorine content could be made, enhancement of these properties would be achieved. One way to increase polymer fluorine content is to prepare dipolymers in which tetrafluoroethylene (TFE) replaces some of the vinyl fluoride (VF). Such dipolymers were first described by Coffman and Ford, U.S. Pat. No. 2,419,009 (1947) and were produced with a benzoyl peroxide initiator in the presence of vinyl fluoride (VF) monomer and water in a high pressure bomb operated at 122-143 atmospheres pressure. This process produces polymer with nonionic phenyl end groups.
Sianesi and Caporiccio, J. Polymer Sci., Part A-1, 6, (1968) 335, and U.S. Pat. No. 3,513,116 (1970) studied the VF/TFE polymerization process in some detail. In all their examples the initiators used by Sianesi and Caporiccio were organometallic compounds whose decomposition was catalyzed by an oxidizing agent to yield alkyl radicals which initiated polymerization. Alcohols or alcohol/water mixtures were commonly used as the polymerization medium in these processes which afforded copolymers at pressures as low as 1 atmosphere. The initiating alkyl radicals of this process result in nonionic end groups on the polymers.
Stilmar, U.S. Pat. No. 3,531,441 (1970), reported preparation of tri- and tetra-polymers composed of VF, TFE and a wide variety of other vinyl monomers by using organic peroxide initiators in different organic solvents as polymerization media. The initiating radicals from the peroxide initiators formed nonionic end groups on the copolymers. Thus, all known previous work has reported the preparation of VF/TFE copolymers, by using radical generating species that place nonionic end groups on the polymer chains formed. These nonionic end groups are typically alkyl or aryl and, therefore, hydrophobic in nature.
The polymerization processes and attendant copolymers produced by the above prior art methods all have some important disadvantages. The process of Coffman and Ford requires high pressures. This demands expensive, robust high-pressure equipment for manufacture of the copolymer. Additionally, since the initiator is nonionic, the copolymer produced is not wetted by water and forms a lumpy heterogeneous mixture in the reactor. Complete removal of the polymer from the reactor is difficult and can only be accomplished by opening the vessel and manually removing it. This is a slow process with some associated danger and is impractical on all but small scale laboratory equipment.
The processes of Sianesi and Caporiccio require the use of flammable alcohol solvents and toxic organometallics such as tetraethyllead which pose some severe health and environmental risks. The polymerization rates reported for these processes are also low and would force the use of very large scale equipment. These requirements demand appropriate plant design for flammable and toxic materials as well as solvent recovery and refining equipment which serve to drive up equipment and operating costs and complicate the overall process.
Cook, et al. U.S. Pat. No. 3,428,618 (1969) teach the use of cyclic azoamidine free radical initiators in a process for polymerizing fluoroolefins. Although they allude to the preparation of dipolymers and terpolymers of fluoroolefins, examples in the patent are directed to the polymerization of VF homopolymer with cyclic 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethylene-isobutyroamidine) dihydrochloride in preference to acyclic 2,2xe2x80x2-azobis(isobutyroamidine)dihydrochloride. Reference to or examples of other homopolymers or interpolymers, specifically VF/TFE interpolymers, is lacking. There is no recognition of producing a superior interpolymer product containing ionic end groups which product has a small uniform particle size and improved weather resistance, chemical resistance, and stain resistance coupled with improved release properties.
The present invention provides interpolymers containing polymer units derived from 1-99 mol % vinyl fluoride and 99-1 mol % of at least one highly fluorinated monomer, which interpolymers are characterized by the presence of ionic end groups. Further, the invention provides aqueous dispersions of the interpolymers comprising 5 to 30% by weight interpolymer and 95 to 70% by weight water. Further provided are processes for preparing the interpolymers by polymerizing VF and fluorinated monomer in water with water-soluble free-radical initiators, preferably azoamidine initiators or persulfate initiators, at temperatures within the range of 60 to 100xc2x0 C. and reactor pressures within the range of 1 to 12 MPa (145 to 1760 psi). Nonaqueous dispersions of the interpolymer in polar organic solvents are also provided in accordance with the invention. The invention further provides protective self-supporting films and coatings formed from nonaqueous dispersions of the interpolymers, both for use on the surfaces of metal, plastic, ceramic, glass, concrete, fabric and wood substrates.
The compositions of this invention comprise an interpolymer of vinyl fluoride and at least one highly fluorinated monomer, which interpolymer is characterized by ionic end groups. The interpolymer has from 1-99 mol % vinyl fluoride and 99-1 mol % of at least one highly fluorinated monomer.
For the purposes of the present invention, xe2x80x9chighly fluorinatedxe2x80x9d means that 50% or greater of the atoms bonded to carbon are fluorine excluding linking atoms such as O or S. Preferred highly fluorinated monomers useful for this invention include fluoroolefins such as those having 2-10 carbon atoms. Preferred monomers also include fluorinated dioxoles, and fluorinated vinyl ethers such as those having the formula CY2xe2x95x90CYOR or CY2xe2x95x90CYORxe2x80x2OR wherein Y is H or F, and xe2x80x94R and xe2x80x94Rxe2x80x2 are independently completely-fluorinated or partially-fluorinated alkyl or alkylene groups containing 1-8 carbon atoms and are preferably perfluorinated. Preferred xe2x80x94R groups contain 1-4 carbon atoms and are preferably perfluorinated. Preferred xe2x80x94Rxe2x80x2xe2x80x94 groups contain 2-4 carbon atoms and are preferably perfluorinated. The most preferred fluoroolefins for use in the present invention have 2-6 carbon atoms and include tetrafluoroethylene (TFE), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), vinylidene fluoride, trifluoroethylene, hexafluoroisobutylene, and perfluorobutyl ethylene. The most preferred dioxoles include perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD).
A preferred composition is an interpolymer which consists essentially of 10-90 mol % vinyl fluoride units and 90-10% tetrafluoroethylene units, preferably 70-10 mol % tetrafluoroethylene units, most preferably 40-20 mol % tetrafluoroethylene units, also characterized by the presence of ionic end groups. For the purposes of the present invention, xe2x80x9cConsists essentially ofxe2x80x9d means that, while the interpolymer may contain other polymer units, the significant properties of the interpolymer are determined by the named monomer units.
Preferred interpolymers in accordance with the invention have a melt viscosity in the range of 100-1000 Paxc2x7s as determined by the method described herein below. Preferred interpolymers are substantially random interpolymers. The substantially random character of the polymer is indicated by nuclear magnetic resonance spectroscopy.
The process of the present invention is directed to the preparation of interpolymers, including dipolymers, terpolymers and higher interpolymers, of vinyl fluoride and other highly fluorinated monomers by emulsion polymerization using water-soluble free-radical initiators. The reactions employed in the preparation process provide good polymerization rates at moderate pressures in water and yield well-dispersed aqueous VF/fluoroolefin interpolymer latices, preferably VF/TFE or VF/TFE modified with additional fluoroolefin monomers.
The interpolymers of this invention are prepared by polymerizing VF and fluorinated monomer in water with a water-soluble free-radical initiator at 60 to 100xc2x0 C. and reactor pressures of 1 to 12 MPa (145 to 1760 psi).
The initiators form ions upon dissolution in aqueous medium, and they introduce ionic end groups into the interpolymers produced. These end groups are derived from initiator fragments which begin the polymerization process. The amount of ionic end groups present in the polymer product is generally not more than 0.05 weight %. Small spherical particles are formed that remain well dispersed in water because of the electrostatic charge on the particle surface arising from the ionic end groups. The electrostatic charge on the particles causes them to repel one another and keeps them suspended in water producing low viscosity interpolymer latices. As a consequence, the latices are fluid and stable enough to be pumped through equipment making the polymerization process easy to operate and control. The invention provides aqueous dispersions of the interpolymers comprising 5 to 30%, preferably 10-15%, by weight interpolymer and 95 to 70%, preferably 90 to 85%, by weight water. Such dispersions can be made more concentrated if desired using techniques which are known in the art.
Initiators useful in this invention are water-soluble free-radical initiators, preferably water-soluble organic azo compounds such as azoamidine compounds which produce cationic end groups or water-soluble salts of inorganic peracids which produce anionic end groups. Most preferred organic azoamidine initiators include 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride and 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyroamidine) dihydrochloride. Most preferred water-soluble salts of inorganic peracids include alkali metal or ammonium salts of persulfate.
For example, 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride produces a copolymer with an amidinium ion as an end group and yields copolymer particles with a positive or cationic charge. Similarly, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyroamidine) dihydrochlioride produces a copolymer with an N,Nxe2x80x2-dimethyleneamidinium ion as an end group and yields positively charged or cationic particles. Persulfate initiators place sulfate end groups on the copolymers which yield negatively charged or anionic particles.
Optionally, as well known to those skilled in the art of emulsion polymerization, additional ingredients may be added to the polymerization medium to modify the basic emulsion process. For example, surfactants compatible with the end groups of the polymer are advantageously employed. For instance, perfluorohexylpropylamine hydrochloride is compatible with the cationic end groups present in polymer initiated by bisamidine dihydrochloride; or ammonium perfluorooctanoate or perfluorohexylethane sulfonic acid or its salts can be used with the polymer having anionic end groups initiated by persulfate salts. As known in the art, reducing agents such as bisulfites, sulfites and thiosulfates may be used with persulfates to lower initiation temperatures or modify the structure of the polymer ionic end group. Buffering agents, such as phosphates, carbonates, acetates and the like, may be used with persulfate initiators to control latex pH. The preferred initiators are the azobisamidine dihydrochlorides and ammonium persulfates used in combination with a surfactant, since they produce the whitest copolymers.
The presence of the amidine hydrochloride end groups in the copolymers of this invention is evident from their infrared spectra. The amidine hydrochloride end group in 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride absorbs at 1680 cm-1. The presence of this end group in the copolymers is confirmed by the appearance of a band in their infrared spectra at 1680 cmxe2x88x921.
The ionic end groups on interpolymers of the present invention also play a role in nonaqueous dispersions. To be useful as protective surfaces, VF/TFE copolymers are fabricated into films and coatings. This is often done by making a dispersion of the copolymer in a latent solvent which does not dissolve the copolymer at ambient temperatures but which reduces the temperature needed to form continuous films. This greatly facilitates manufacture of thin films 10 xcexcm to 100 xcexcm thick by easily permitting a dispersion of the copolymer to be thinly spread on a substrate at ambient temperatures. The coated substrate is then heated, at which point the latent solvent coalesces the copolymer particles into a continuous solvent-swollen gel, and then evaporates leaving a dry defect-free film. Obviously, dispersion rheology will play an important part in determining the quality of the coating or film produced. It is well known in colloid science that particle size, shape and surface energy are important factors governing dispersion rheology.
The useful latent solvents are generally polar organic liquids like propylene carbonate, N-methyl pyrrolidone, xcex3-butyrolactone, dimethylacetamide, dimethylsulfoxide and the like which have surface tensions near or above 35 dyne/cm. The surface tension of PVF and VF/TFE interpolymer films have been measured and it is found that the surface tensions decrease from 38 dyne/cm for PVF to about 26 dyne/cm for interpolymers containing 40 mol % TFE. The presence of ionic end groups on the interpolymer particle surface increases the surface energy of the particle so that it is wetted by the coalescing solvent, thus aiding in their dispersion. There may also be some electrostatic stabilization of these dispersions arising from the surface charge. In addition, the interpolymer particles of the present invention are submicrometer in size, preferably having an average diameter of 200 to 400 nm. The particles also preferably have a narrow uniform size distribution. Preferably, at least 90% of the interpolymer particles are within a range between plus or minus 10% of the average particle diameter, and more preferably, between plus or minus 5% of the average particle diameter. The interpolymer particles are preferably substantially spherical in shape. By substantially spherical is meant that the ratio of the average diameter of the particles along the major and minor axes is from 1.0:1 to 1.3:1. This morphology will allow high concentrations of interpolymer particles to be dispersed in the latent solvent while maintaining viscosities convenient for coating. In summary, the interpolymer particles with ionic end groups have a good balance of properties which facilitate the preparation of interpolymer dispersions. This allows production of uniform, low viscosity dispersions of the interpolymer particles which can then be used to coat articles or cast high quality thin films. Typically, viscosities measured for dispersions in latent solvents containing 20-40 wt. % polymer particles at 25xc2x0 C. with a cone and plate rheometer at a shear rate of 2 per second will be in the range 500 to 10,000 centipoise (0.5 to 10 Paxc2x7s), preferably 1500 to 3500 centipoise (1.5 to 3.5 Pxc2x7s).
The absence of ionic end groups on the interpolymers of the prior art produce particles with low surface energy that are poorly wetted by the solvent. As a consequence, they tend to agglomerate when dispersed in latent solvents and produce coatings or films with defects. Interpolymers with nonionic phenyl end groups prepared by the method of Coffman and Ford produce interpolymer particles which vary in size from submicrometer to greater than 10 xcexcm. The particles have irregular shapes and often contain channels and voids. As a consequence, particles can interlock with one another affecting dispersion viscosity. Films cast from these dispersions have a very undesirable appearancexe2x80x94being discolored and full of defects known as craters.
As is evident by the above discussion, the vinyl fluoride interpolymer compositions of this invention can be used to make high quality protective films and coatings on a wide variety or substrates such as metal, plastic, ceramic, glass, concrete, fabric and wood. Techniques for producing such films include conventional methods of casting, dipping, spraying and painting. Normally films are deposited as wet dispersions or solutions and subsequently dried and coalesced thermally. Dispersions of this invention are also suitable for preparing powder coatings, for example, spray drying a dispersion will produce powder particles that can be electrostatically sprayed onto a substrate and fused to form a uniform coating.
Polymer Composition
Polymer composition was determined by 19F-nmr measuring the spectrum at 235.4 MHz of each polymer dissolved in dimethylacetamide at 130xc2x0 C. Integration of signals near xe2x88x9280 ppm arising from CF3 groups was used to measure the amount of hexafluoropropylene (HFP), perfluorobutyl ethylene (PFBE) or perfluoro(ethyl vinyl ether) (PEVE) when present in the polymer. Integration of complex sets of signals from xe2x88x92105 to xe2x88x92135 ppm for CF2 groups from TFE units in the interpolymer, corrected for the CF2 content contributed by any other monomer when present, and from xe2x88x92150 to xe2x88x92220 ppm for CHF groups from the VF units in the interpolymer corrected for the CF content contributed by any other monomer when present provided complete compositional data for each sample.
For interpolymers containing TFE and CTFE where F-nmr signals for TFE and CTFE are complex multiplets that strongly overlap, nmr analysis alone could not determine the individual amounts of these components in the polymer. To fully resolve the composition, a mole ratio of TFE to CTFE was assumed and that, along with the nmr integrals for the combined TFE and CTFE and for the VF in the polymer, was used to calculate a mole percentage composition for each monomer component of the polymer. The calculated chlorine weight percentage for this composition was then compared to chlorine weight percentage determined by elemental chlorine analysis. The difference between the calculated and measured chlorine content was used to refine the estimate of the TFE to CTFE mole ratio and the monomer composition of the polymer was recalculated. This interative process was repeated until the calculated and measured chlorine contents converged on the same value giving the correct amount of each monomer in the polymer.
Infrared spectroscopy was used to identify the presence of ionic end groups.
Melt Viscosity
Melt viscosity was used as a relative measure of molecular weight. The melt viscosities of some of the polymers were extremely high and exhibited extreme melt fracture when extruded from the die of the capillary rheometer. This gave erratic results. In order to obtain a reliable measurement, mixtures of 40 weight % polymer in dimethylacetamide were used with the rheometer operated at 150xc2x0 C. Viscosities were measured at shear rates ranging from 23.4 secxe2x88x921 to 3516 secxe2x88x921. Logarithmic plots of these data were fit to a linear least squares equation which was then used to calculate the viscosity at 100 secxe2x88x921. This value is reported as MV.
Melting Point
Melting points (Tm) were measured in air by differential scanning calorimetry (DSC). Because the thermal history of the sample can affect the measurement of Tm samples were heated to 250 xc2x0 C. at 10 xc2x0 C./min, then cooled and reheated at 10 xc2x0 C./min. The peak temperature of the endotherm observed during the reheat of the sample is reported as Tm.
Color
The color of each sample was measured with the aid of a Hunter Lab UltraScan spectrocolorimeter (Hunter Associates Laboratory, Reston, Va.) and compared to a standard white sample to obtain xcex94L, xcex94a and xcex94b values.
Accelerated Weathering Tests
Accelerated weathering tests were conducted according to test protocol specified in standard SAE J1960. The test is a method to gauge the performance of materials exposed to extreme environmental conditions due to sunlight, heat and moisture in the form of humidity, condensation or rain.
Heat of Fusion
Heat of fusion of the polymer was determined by integrating the area under the melting endotherm recorded by the DSC and is reported as xcex94Hf in J/g.
Haze Measurement
Haze measurements were made on film with a Hunter Lab Ultra Scan Spectrocolorimeter manufactured by Hunter Associates Laboratory, Reston, Va. operated as described in the instrument instruction manual. Haze is reported as the percent diffuse transmission of total light transmission for the sample and provides an index of light scattering by the sample.
Surface Tension
Film surface tension measurements were made by the harmonic mean method of Wu (S. Wu, J. Poly. Sci., Part C, (34), 19, (1971)) using water and methylene iodide as the probe liquids.
Dispersion Viscosity
Dispersion viscosities were determined using a Brookfield cone and plate viscometer, Model DV-1 supplied by Brookfield Engineering Laboratories of Stoughton, Mass., operated according to manufacturer""s instructions.